Homogeneous incorporation of activator element in a storage phosphor

Abstract

A method has been disclosed for manufacturing a storage phosphor for use in a photostimulable phosphor screen or panel comprising a support and a storage phosphor layer, wherein a dopant or activator is incorporated more homogeneously in amorphous and in crystalline phosphors as well, starting with a mixing step of said matrix component and activator component in stoechiometric ratios in order to provide a desired phosphor composition; and more particularly in order to prepare a CsBr:Eu 2+ phosphor having an optimized sensitivity with respect to its particle size.

1. Method of preparing a phosphor comprising the step of growing its surface by firing or evaporating as components composing the said phosphor, a matrix component and a dopant activator element, wherein growing said phosphor, during a last incremental growth before ending said growth, proceeds over a distance q, expressed in μm, in its largest direction, provided that an increase of the said phosphor in volume, expressed in volume percentage, is more than 5%; characterized in that an activator element partition, determined before and after growth of a surface layer of said phosphor over a distance corresponding with q/10 satisfies the condition:
   −( q/ 10) −1 <rico<0 
 
     wherein said “rico” value, expressed in μm −1 , is calculated as a ratio of
 concentration differences of said activator concentration (1), determined at a depth of q/10 under said surface and said activator concentration (2) determined at said surface, and 
 said activator concentration (2) determined at said surface, 
 
     divided by the product of 0.1 times q. 
   
   
     2. Method according to  claim 1 , wherein q equals a value of 50 μm for needle-, prismatic-, cylindrical- or block-shaped phosphor crystal and wherein its largest direction is its height direction. 
   
   
     3. Method according to  claim 2 , comprising as preparation steps:
 mixing said matrix component and a component containing said dopant activator element in stoechiometric ratios in order to provide a desired phosphor composition; 
 milling or grinding said matrix component and dopant activator component; 
 putting a mixture of said matrix component and said component containing said dopant activator element in an inert crucible in an apparatus providing reaction under a reduced pressure atmosphere; 
 firing said mixture up to a temperature T, at least equal to or higher than the melting temperature T melt  of the desired phosphor; 
 cooling the said phosphor. 
 
   
   
     4. Method according to  claim 3 , wherein the said firing is performed in order provoke vapor deposition of the resulting phosphor onto a substrate, said vapor deposition being performed by a method selected from the group consisting of physical vapor deposition, thermal vapor deposition, chemical vapor deposition, electron beam deposition, radio frequency deposition and pulsed laser deposition. 
   
   
     5. Method according to  claim 4 , wherein before or during mixing said matrix component and said component containing said dopant activator element, at least one anti-caking agent is added. 
   
   
     6. Method according to  claim 5 , wherein said anti-caking agent is a compound selected from the group consisting of a silica, a metal oxide, a zeolite and a ceramic compound. 
   
   
     7. Method according to  claim 3 , wherein before or during mixing said matrix component and said component containing said dopant activator element, at least one anti-caking agent is added. 
   
   
     8. Method according to  claim 7 , wherein said anti-caking agent is a compound selected from the group consisting of a silica, a metal oxide, a zeolite and a ceramic compound. 
   
   
     9. Method according to  claim 3 , wherein said reduced pressure atmosphere is in the range of 1 mbar or lower. 
   
   
     10. Method according to  claim 3 , wherein said reduced pressure atmosphere is a reducing atmosphere. 
   
   
     11. Method according to  claim 3 , wherein during or after cooling, an annealing step is performed. 
   
   
     12. Method according to  claim 11 , wherein said annealing step is performed by heating said phosphor in an oxygen-containing atmosphere up to a temperature in the range from 50° C. to 400° C., during a time in the range from 5 minutes to 15 hours, followed by cooling. 
   
   
     13. Method according to  claim 3 , wherein said phosphor is subjected to a further heating step, by heating to a temperature T in the range from 50° C. to 400° C., during a time in the range from 5 minutes to 15 hours, followed by solidifying said phosphor by a cooling step. 
   
   
     14. Method according to  claim 1 , wherein q equals a value of 2 μm for a globular phosphor crystal and wherein its largest direction is its diametrical height direction. 
   
   
     15. Method according to  claim 14 , comprising as preparation steps:
 mixing said matrix component and a component containing said dopant activator element in stoechiometric ratios in order to provide a desired phosphor composition; 
 milling or grinding said matrix component and dopant activator component; 
 firing (calcinating) said mixture up to a temperature T, from T melt −100° C. to T melt +100° C., wherein melting temperature T melt  represents the melting temperature of the desired phosphor; 
 cooling the said phosphor. 
 
   
   
     16. Method according to  claim 15 , wherein before or during mixing said matrix component and said component containing said dopant activator element, at least one anti-caking agent is added. 
   
   
     17. Method according to  claim 16 , wherein said anti-caking agent is a compound selected from the group consisting of a silica, a metal oxide, a zeolite and a ceramic compound. 
   
   
     18. Method according to  claim 15 , wherein after cooling said phosphor is subjected to a grinding or milling step up to a desired average particle size. 
   
   
     19. Method according to  claim 18 , wherein said grinding or milling step is performed in an Alpine mill or a planetory mill. 
   
   
     20. Method according to  claim 18 , wherein in said grinding or milling step an organic solvent is added. 
   
   
     21. Method according to  claim 18 , wherein in said grinding or milling step a dispersing agent is further added. 
   
   
     22. Method according to  claim 1 , wherein components composing the said phosphor are
 a matrix component (1−a)M I X.aM II X 2 , wherein 
 M(I) is at least one of monovalent Li, Na, K, Rb or Cs, 
 M(II) is a divalent metal element selected from the group consisting of Mg, Ca, Sr, Ba and Ni; 
 X is a halogen atom selected from the group consisting of F, Cl, Br and I; wherein 0≦a<0.5; and 
 a dopant activator element Ln, wherein 
 Ln stands for Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Bi, In or Ga. 
 
   
   
     23. Method according to  claim 1 , wherein said matrix component is CsBr and wherein said dopant activator element is Eu. 
   
   
     24. Method according to  claim 23 , wherein said dopant activator element Eu is generated from phosphor precursors selected from the group consisting of EuX 2 , EuX 3 , EuOX and Cs x Eu y X x+αy , wherein x/y>0.25, and wherein α≧2, and wherein X is a halide selected from the group consisting of Cl, Br and I and combinations thereof. 
   
   
     25. Method according to  claim 23 , wherein said Eu is generated from EuBr 3  and is present in the CsBr:Eu phosphor in a divalent state in an amount of at least 99.9 mole %.